Obesity is one our nation's most serious public health problems. Weight losses as little as 5-10 percent of initial weight are frequently sufficient to improve health complications of this disorder. Most obese individuals are able to achieve reductions of this size by following a program of diet and lifestyle modification. The great majority, however, regain one-third of their weight loss in the year following treatment, with increasing regain over time. Evidence from a variety of studies indicates that regular exercise is associated with improved maintenance of weight loss, as well as with enhanced fitness and health. Despite these benefits, obese (and average weight) individuals report numerous barriers to exercise, including inadequate time and beliefs that it is associated with exhaustion, physical discomfort, and potential embarrassment. As a result, adherence to structured exercise programs is far from optimal. Lifestyle activity may be a preferable alternative to traditional exercise. It offers a more flexible approach to increasing physical activity by increasing walking throughout the day, taking stairs rather than escalators, and generally decreasing reliance on energy-saving devices. Two studies of children showed that lifestyle activity was superior to structured exercise in facilitating the maintenance of weight loss. The goal of the proposed research is to improve the maintenance of weight loss by increasing physical activity in individuals who participate in a comprehensive behavioral weight loss program. A total of 216 obese adults (108 males, 108 females) will be randomly assigned to one of three conditions: 1) structured on- site exercise; 2) structured at-home exercise; or 3) lifestyle activity. Subjects will be treated for 40 weeks and then followed through week 104. We predict that, at week 104, subjects in the lifestyle condition will maintain their end-of- treatment weight losses significantly better than those in the two other conditions and will have significantly greater improvements in health and psychosocial status. This will result from their maintaining significantly higher levels of physical activity during the follow-up period. The predicted superiority of lifestyle activity is based on extensive pilot data and, if confirmed, should significantly improve the management of obesity and its associated health complications.